1. Field of the Invention
This invention generally relates to optical metrology apparatus and more particularly to apparatus and methodology for aligning an image forming optical system to a video metrology tool.
2. Background of the Prior Art
To properly measure an optical system, one needs to bring its optical axis into alignment with the optical axis of a collimated beam provided in the metrology tool. While this requirement is easily stated, co-alignment of the optical axes of the test instrument and the unit under test (UUT) is usually an awkward and time consuming process in practice, typically achieved by putting a mirror on a reference surface of the UUT and attempting to reflect the incident collimated beam back onto the collimator target. The return image formed at the target is usually difficult to see even at visible wavelengths and is not visible to the human eye if in the infrared or deep ultraviolet.
Well-known metrology instruments contain a light source, test target, refractive or reflective collimator, and an image analyzer. The image analyzer is generally comprised of a relay lens and two-dimensional video sensor, such as a CCD camera for the visible spectrum or microbolometer for the long-wave infrared (LWIR) spectrum. The optical system to be tested (unit-under-test or UUT) forms an image of the illuminated test target at an infinite conjugate. The image analyzer captures this image for analysis to determine properties and qualities of the UUT. An example of such instruments is described in detail in U.S. Pat. No. 5,661,816 which issued on Aug. 26, 1997 in the name of Stephen D. Fantone, et al. with the title “IMAGE ANALYSIS SYSTEM.”
To properly characterize the properties of a UUT with a metrology tool requires that it be aligned with the collimated beam of the metrology tool such that there is no tilt between the optical axes of the UUT and that of the metrology tool.
Accordingly, it is a primary object of the present invention to provide alignment apparatus and methodology by which an optical component to be measured in a video based metrology instrument can readily be aligned with respect to the other components comprising the system.
Other objects will be obvious and others will appear hereinafter when the following detailed description is read in connection with the accompanying drawings.